Water mattress bag, and a method for producing the same

ABSTRACT

A water mattress bag (1) of a watertight resilient material with tensile strength is provided with a valve for filling/draining water and is intended for use together with a main mattress member (3) of a resilient, flexible foamed plastic or the like, which is provided with a recess (4) in its upper face extending over a substantial portion of its length (L) and width (B). The bottom (1a) of the bag (1) is shaped with a bulge (1a&#39;) for cooperation with recess (4), and the top (1d) of the bag (1) has a length and width approximately equal to the length (L) and width (B). 
     Water mattress (10) comprises the above mentioned components and a flexible sheet (15) on top of water mattress bag (1) and a mattress cover (16). Water mattress bag (1) is partly filled with water and extends outwards to the lateral edges (3a) of main mattress member (3) and covers the flat edges (3b) of main mattress member (3) around recess (4). Sheet (15) is a weight distributing sheet and shaped complementary with the opening of recess (4), and has a thickness (a) causing the top (15a) of weight distributing sheet (15) to project higher than said flat edges (3b) of main mattress member (3) in a non-loaded state. Mattress cover (16) is stretched around main mattress member (3) with components (1,15) to provide a bias/increase of pressure of the water in water mattress bag (1) in an un-loaded state of the mattress (10).

The present invention relates to a water mattress bag for use with amain mattress member of the kind as defined in the introductory part ofthe following independent claim 1. Furthermore, the invention relates toa method for producing such a water mattress bag. The invention alsorelates to a water mattress as defined in the introductory part of thefollowing claim 8.

Conventional water beds comprise a sturdy bottom plate with framemembers provided on a weight distributing base. Said frame membersenclose a water bag provided on said bottom plate and consisting of aflexible material with tensile strength, and containing approximately400-500 liters of water for each person unit, resulting in a water depthof approximately 20-24 cm in a bed area of normal size. Said framemembers which must necessarily project above the level of said water bagdo not provide for comfortable sitting on the edge of the bed, and thebearing capacity of said water bag is highly reduced adjacent the insideof said frame. The large volume of water will result in disadvantages,e.g. high weight, great hazard of water damages in case of a leakage,and high operation costs for heating water and supplying antialgalagents to prevent growth. In order to prevent water leakages in suchwater beds a safety sheet is provided between said water bag and thebottom and frame memebers. Due to the fact that said safety sheet isprovided in direct contact with the water bag there is a hazard ofdouble puncture, i.e. that a pointed or sharp object can puncture bothwater bag and safety sheet at the same place with a resulting leakage ofwater into the room.

In further developed water bed mattresses the above disadvantages ofconventional water beds were partly eliminated, e.g. as disclosed inU.S. Pat. Nos. 3,689,949 and 3,456,270. These specifications show amattress made of foamed plastic or another suitable material shaped witha recess in the surface that is adapted to the dorsal profile of aperson in a lying position. A bag or the like of a water tight flexiblematerial filled with water is placed on top of said mattress in saidrecess. In this manner a reduction of the weight of the water mattressis achieved by utilizing a light weight mattress material, e.g. foamedplastic, as well as reduced use of water the volume of watersubstantially being present in a recess that is adapted to the humanbody. In case of pin-point load the soft material of said foamed plasticmattress will, furthermore, subdue or cause an elastic contact with theharder support in the bottom of the bed or with any other base on whichsaid water bed mattress is provided. It is, furthermore, known toprovide a cover on top of such a water bed mattress, said cover beingfastened to the side faces of said foamed plastic mattress, e.g. by theaid of hook and loop type fasteners, such as the one sold under thetrademark of Velco.

The last mentioned water bed mattresses are intended for being providedin ordinary beds comprising a bottom and a frame. The water bag itselfis, thus, shaped with top and bottom sides having dimensions that arlarger than those of the frame of the bed, i.e. larger than thedimensions of the frame length and width, so that the bottom face andthe top face of said bag adapt to the surface of said foamed plasticmattress with its recess and to the shape of the user's body resultingin a pressure of said water bag directed outwards toward the insides ofsaid frame.

It is an object of the present invention to provide a water mattress bagfor use with a main mattress member consisting of foamed plastic oranother suitable material and shaped with a recess in its surface, asmentioned above, with said water mattress bag shaped so as to maintainits peripheral shape intact when it is filled and placed on a mainmattress member with a recess, i.e. so as not to need any lateralsupport, and that said peripheral shape is also maintained when saidwater mattress bag is loaded by a person sitting or lying down on it.According to the invention this is achieved by the features appearingfrom the characterizing part of the following independent claim 1 andthe following dependent claims.

Another object of the invention is to provide a method for producing awater mattress bag of the kind mentioned above so as to achieve simpleand inexpensive production. According to the invention this is achievedby the features appearing from the characterizing part of the followingmethod claims.

Water beds gradually developed from being a medical aid used inhospitals and institutions to a very popular piece of sleeping furniturewhich has general use.

Water beds of the original design contain much water causing a greatload of weight on bed and floor which brought about special bedstructures with a weight distributing support on the base, i.e. Thefloor.

In order to produce a water bed mattress for use in ordinary beds andwithout use of a weight distributing support it is necessary to reducethe volume of water to such a degree that the water bed mattress may besupported by conventional bed structures and without any furtherauxiliary means.

Such a reduction of the weight of the water mattress is achieved bytechnology as mentioned above, which is also disclosed in GB-PS No. 1140 767 describing a resilient main mattress member with a recess curvedin the longitudinal and transversal directions in its upper surface, inwhich recess a partly filled water bag is placed.

A resilient sheet provided on top of the water bag, and mainly intendedas a thermal insulation between the person lying on said water mattressand the water in said water bag, is shown in U.S. Pat. No. 4,186,455 andGB-PS No. 1 559 264. The two last mentioned specifications, however, donot show said resilient main mattress member with said curved recess inthe upper surface and, thus, comprise a water mattress bag filled with alarger volume of water than water mattresses of the kind mentionedabove.

Similar water/weight reducing mattress structures are, among others,known from the American mattress model "Somma", foom the Ulfert model"Royal", and, among others, from the Norwegian patent application Nos.845056 and 861319. These structures are based on somewhat differingprinciples, however, it is a common characteristic that they requirelarger volumes of water than the present invention without offeringbetter lying comfort than that achieved by the water mattress accordingto the present invention.

It is a further object of the present invention to achieve a water bedmattress which contains very little water and which only requiresapproximately 50-140 liters per person unit, depending on the mattressdimensions, and which shows subdued waves without any use of a built-inwave-calming means, as well as maintaining its load bearing capacityeven in case of minimum external dimensions, e.g. with a width of 75 cmof the mattress. Furthermore, such a water mattress has dimensions andthe appearance of an ordinary mattress, and it is inexpensive inproduction.

According to the present invention this is achieved by thecharacterizing features stated in the following claim 8 and in thefollowing dependent claims 9-13.

Embodiments of the invention are disclosed in more detail below withreference to the drawing, where

FIGS. 1 and 2 show views in perspective of a water mattress bag in afilled state and a foamed plastic mattress with a recess and with acovering sheet and a safety sheet partly removed to facilitateunderstanding, respectively,

FIG. 3 shows a blank of a water mattress bag in a flat state,

FIG. 4 shows the bag blank welding completed with longitudinal andtransversal welded seams,

FIG. 5 shows the same blank with folded end portions,

FIGS. 6 and 7 show mattress bag with folded end portions in alongitudinal section, and a cross section, respectively, with the shapesaid bag will have when filled with water and placed in the associatedmain mattress member,

FIG. 8 shows the components of a water mattress in an exploded view inperspective,

FIG. 9 is a cross sectional view of the water mattress in an unloadedstate, and without a mattress cover,

FIG. 10 shows the same mattress in a loaded state,

FIG. 11 shows the same cross sectional view as FIG. 9, but with astretched mattress cover, and

FIG. 12 shows a water mattress with a stretched cover according to FIG.11, but in a loaded state.

FIG. 1 shows a water mattress bag 1 of a resilient material havingtensile strength and provided with a valve 2 for filling up, anddraining off water, respectively. Said water mattress bag has a bottomface 1a provided with a bulge 1a' and a top face 1d which is plane andhas a length and a width corresponding to length L and width B of a mainmattress member 3, as shown in FIG. 2, e g. made of resilient flexiblefoamed plastic. The main mattress 3 has recess which extends over asubstantial part of its surface in the longitudinal and transversaldirections. Said water mattress bag 1 is, thus, intended for lying--in aliquid filled state--on the surface of said main mattress member 3 andto rest with its bulge 1a' in said recess 4 of the mattress surface. Thewater bed mattress thus formed is preferably provided with a safetysheet 7 of watertight material which extends beneath the bottom of saidmain mattress member 3 and alongside its lateral edges, and acts as aleakage barrier, as shown in FIGS. 1 and 2. On top of water mattress bag1 a cover 8 is preferably provided and extends down over the lateraledges of the main mattress member 3 outside said safety sheet 7, and maybe fastened to the latter, e.g. by hook and loop type fasteners, such asthe one sold under the trademark of Velcro or in any other desirablemanner.

By use of a foamed plastic mattress having a thickness of approximately17 cm and being provided with a cavity forming said recess 4, a basinwhich can receive a water mattress bag 1 only containing approximately150 liters may be achieved by suitable dimensioning of the length,width, and depth of said recess 4. This constitutes an essentialreduction of. The volume of water as compared with previously knownwater bed mattresses. Due to the design of the water mattress bag 1 themain portion of the volume of water will be in recess 4 and with a waterdepth of approximately 12 cm, provided that recess 4 is approximately 12cm deep. This water depth is sufficient to carry a lying person withoutsaid person contacting the bottom of recess 4. In case of a pin-pointload, i.e. when a person, e.g. sits on water mattress bag 1, the loadwill be transmitted in a subdued manner to the support of main mattressmember 3 via the bottom of recess 4 which is approximately 5 cm thick.

Water mattress bag 1 preferably consists of a blank A of a watertight,resilient foil/sheet material showing tensile strength, as shown in FIG.3, with two pairs of opposite edges 1b,1b, and 1c,1c. One pair oflateral edges 1b,1b is provided with curved or tongue shaped projections1b',1b' extending along a substantial portion, of the lateral edges1b,1b and ending at a distance from the said transverse edges 1c,1c.When water mattress bag 1 is produced, said lateral edges 1b,1b withprojections 1b',1b' are placed in a level state one on top of the otherand are joined, e.g. by lap welding with a longitudinal welded seam 5being formed. In this manner a sleeve A' is shaped from blank A with itsopenings limited by the second pair of lateral edges 1c,1c. The endopenings of sleeve A' are then closed by transverse welded seams 6, 6 inthe end portions of sleeve A' that were placed in a flat state. Whensaid end portions are placed in a flat state it is necessary to ensurethat said first longitudinal lateral edge joined/welded seam 5 willcross said transverse welded seams 6, 6 in their central portion. Theprojection or bulge 1a' formed due to the curved projections 1b',1b' ofthe lateral edges 1b, 1b will, thus, be oriented under water mattressbag 1 and form its bottom 1a, whereas the opposite portion of closedsleeve A' will form the top face 1d of said water mattress bag which hasa rectangular shape corresponding to the contour of main mattress member3. In order to improve the longitudinal bulge of the bottom 1a of watermattress bag 1 said bottom and top faces may be mutually displaced as totheir longitudinal direction by the aid of said transverse welded seams6,6.

As shown in FIG. 4, said transverse welded seams 6, 6 are placed at adistance from the ends of sleeve A' and at a mutual distance that is,preferably, a little larger than length L of main mattress member 3.Thus, the end portions of water mattress bag 1 with said transversewelded seams 6, 6 may be folded in under said water mattress bag when itis placed on top of main mattress member 3 with said transverse weldedseams 6, 6 in the folded portions. Sudden increases of pressure insidewater mattress bag 1, e.g. when a person lets himself fall onto thewater bed mattress, will then be subdued at the fold, and transversewelded seams 6, 6 will receive a reduced load from the pressure tryingto force the lap seams apart.

FIGS. 6 and 7 show a longitudinal section, and a cross section,respectively, of water mattress bag 1 having a plane top face 1d and anunderside with a bulge 1a'. In FIG. 6 the end portions of said watermattress bag are shown in a folded state.

The disclosed water mattress bag 1 has a very simple structure and may,thus, be produced in a simple and inexpensive manner since there areonly three simple lap-welded seams 5, 6, 6, to be made, i.e. said watermattress bag is only welded in three places.

This is done by folding said blank A so as to bring its opposed lateraledges 1b, 1b with their curved projections 1b',1b' in contact and bylap-welding to obtain the resulting lap-welded seam 5. The end portionsof sleeve A' formed in this manner are then placed in a flat manner withwelded seam 5 approximately in the center of the formed face. Said endportions of sleeve A' are then closed by lap-welding 6, 6 across saidend portions at a distance from their ends that are limited by saidopposed lateral edges 1c, 1c of the blank.

As will appear from the above described water mattress bag, there are noexpensive and complicated corner designs with associated leakageproblems. According to the method for producing said water mattress bagthe top face and underside of the bag lie flat in contact in the joiningareas, and they can be joined by the aid of simple lap-welding.

In stead of the above mentioned lap-welding, obviously, any suitablemethod of joining the lateral edges 1b,1b of bag blank A and of closingthe obtained sleeve A' at its end portions may be used.

The above mentioned mutual displacement of the bottom and top faces 1a,1d of said water mattress bag 1 in order to obtain an improvedlongitudinal bulge of the bottom is achieved by displacing the endportions of bottom side 1a towards each other, whereas top face 1d isleft as it is. Then the pairs of top faces 1d and bottom faces 1a arejoined, e.g. by said lap-welding to form lap welded seams 6, 6. Thus,the bottom face 1a will become longer than top face 1d.

In order to ensure that a correct displacement is achieved in connectionwith welding said transversal welded seams 6, 6 the blank A may, inadvance, be provided with marking lines 6' which lines extend in awavelike shape across the end portion of blank A. Said wavelike shape ismade so as to form the largest distance between marking lines at each ofthe longitudinal lateral edges 1b, 1b of blank A and the shortestdistance at the longitudinal center of blank A. When blank A has beenfolded and welded along its longitudinal edges 1b, 1b those portions ofmarking lines 6' extending on top face 1d and bottom face 1a,respectively, are aligned by displacement of bottom face 1a before thelap weld is made which will, thus receive a curved shape.

In order to reduce the load on the end portions of the lap welded seamin case of a sudden increase of pressure inside water mattress bag 1,the end portions of transverse welded seams 6 are curved toward eachother to change softly into the longitudinal sides of water mattress bag1.

For the production of a water bed mattress 10 according to the presentinvention a main mattress member 3 of foamed plastic, cold setting foam,latex, foamed rubber, polystyrene, or the like is used. In its uppersurface a recess 4, a so called lying cavity is cut or moulded, saidcavity being concavely curved both in the longitudinal and thetransversal direction, adapted to the contours of a lying person in sucha manner that the concave longitudinal curve is less curved under theleg/foot portion and more curved under the head end. In said recess 4 awater mattress bag 1 of the above mentioned kind which is adapted to thedimensions of said recess is placed. Said water mattress bag has abottom 1a the shape of which is adapted to recess 4, and a flat top 1d.Water mattress bag 1 is partly filled with water and extends to thelateral edges 3a of main mattress body 3 and, thus, covers the flatedges 3b of main mattress member 3 around recess 4, the top 1d of watermattress bag essentially being lower than said flat edges 3b.

If recess 4 is cut out of main mattress member 3, a correspondinglyshaped piece of material is left, which fits accurately in said recess.By cutting a top piece with a thickness of, e.g. 4 cm a resilient sheet15 will result with a contour complementary to the recess 4 opening.This sheet may be used as a weight distributing sheet 15 on top of watermattress bag 1. Obviously, any desirable sheet of a similar material andwith a corresponding contour may be used for a weight distributingsheet.

The thickness of weight distributing sheet 15 is such that its upperside 15a in an unloaded state will at least project to the flat edges 3bof main mattress member 3, but preferably somewhat higher. A mattresscover 16 is stretched around main mattress member 3 with the abovementioned components 1 and 15 to keep said components in place, and topprovide a bias/increase of pressure in the water in water mattress bag 1when water mattress 10 is in an unloaded state. Between said mattresscover 16 and weight distributing sheet 15 a resilient sheet 17 is,preferably, arranged to extend to the lateral edges 3a of main mattressmember 3, said sheet 17 being softer than said weight distributing sheet15.

With such a build-up of the water mattress according to the inventionthe water inside said water mattress bag 1 is kept under a moderatehydrostatic pressure by the aid of said stretched mattress cover 16.

The material of said water mattress bag is thus subjected to atensioning load so that part of the resiliency of the material is usedup. Due to said bias compression of the water mattress bag will bereduced resulting in less sag when there is a further load. This meansthat said water mattress can tolerate a greater load without saggingthrough,--i.e. letting the loading body reach the bottom of recess4--with resulting compression of the main mattress member 3 than is thecase with water mattresses without any bias in the water mattress bag.When said water mattress has a load put on it, said weight distributingsheet will distribute said load to a larger area of the water mattressbag, especially in case of a pin-point load, and sagging is thusreduced. On top of weight distributing sheet 15 a softer resilient sheet17, e. g. of foamed plastic, may be provided to extend outwards to thelateral edges 3a of main mattress member 3, in order to provide a softerbed to lie on. Said softer sheet 17 may, if desired, be combined withweight distributing sheet 15 to one member. Such a member may, ifdesired, be produced from foamed plastic materials of different degreesof hardness from the underside to the top. The water mattress comprisingmain mattress member 3, water mattress bag 1, weight distributing sheet15, and said softer sheet 17 on top, and with the last mentioned sheets15 and 17 projecting slightly above the flat edges 3b of main mattressmember 3 around recess 4, as mentioned, is held together by a stretchedmattress cover 16, which is preferably put around said components by theaid of a mattress covering machine in a manner known per se. This meansthat said main mattress member with the mentioned further components isslightly compressed so as to have a slightly smaller width and length.The mattress cover can now be pulled otto said main mattress member, thecompressing means is removed, and mattress cover 16 is now stretchedaround the entire main mattress member with components as mentioned.

Said stretched mattress cover 16 will exert a moderate pressure on thesofter upper sheet 17 which will, in turn, press down onto weightdistributing sheet 15, and the latter will press down onto the top face1d of water mattress bag 1. In this manner said bias of water mattressbag 1 is achieved with the mentioned partial consumption of theresiliency of the water mattress bag material.

When said water mattress is loaded by a lying person the weight of saidperson's body will be distributed by weight distributing sheet 15 anylocal sag into the water being transmitted to a larger area. Due to thebearing capacity of said larger area or surface any sag is reduced andrequires a smaller depth of water beneath said weight distributing sheet15, without causing said water mattress to appear rigid or hard, sinceweight distributing sheet 15 is flexible and will adapt to the movementsof both body and water.

In case of such a load the water in water mattress bag 1 will tend to"overflow" the edge of recess 4 and, thus, form a sausage-like swelling1c--FIG. 10--on top of the flat edges 3b of the main mattress member 3around recess 4. This means that water is forced into the marginal zonesof water mattress bag 1 where there is no water in a non-loaded watermattress.

Such an "overflow" is reduced because mattress cover 16 of mattress 10is least resilient in the upward direction of mattress 10. A watermattress having such a stretched mattress cover 16 will have higherbearing capacity than one without such a mattress cover, because thelateral displacement of water into the marginal zone of water mattressbag 1, which zone may also be called expansion portion, will compressthe uppermost soft sheet 17 in its peripheral edges, as well as part ofthe foamed material in main mattress member 3 surrounding water mattressbag 1. This cooperation between components and materials of the watermattress will cause a synergy effect provoking a "springy" effect of thewater, since the water will always ee forced back with a higher forcedue to said bias than if it flows back of its own gravity alone.

The above mentioned features are illustrated in FIGS. 9-12 in thedrawing. FIG. 9 shows said main mattress member 3 with a partly filledwater mattress bag 1 in recess 44,and with weight distributing sheet 15and the softer sheet 17 on top in a non-loaded state without anymattress cover 16. It will clearly appear that weight distributing sheet15 lies on the top 1d of water mattress bag 1 and has a thickness acausing weight distributing sheet 15 to project over the flat edges 3bof main mattress member 3 around recess 4.

If the water mattress as shown in FIG. 9, without any stretched mattresscover, is loaded, said weight distributing sheet 15, and the softerupper sheet 17 will be pressed down by the body of a lying person, asshown in FIG. 10, toward water mattress bag 1 which will be compressedin its central longitudinal area water being displaced to the marginalzones of said water mattress bag which will, thus, form a "sausage like"swelling 1c that may also, as mentioned above, be called the expansionportion of said bag.

By providing a stretched mattress cover 16 outside the components of thewater mattress as shown in FIG. 9, an increase of the hydrostaticpressure inside water mattress bag 1 is chieved as mentioned above andshown in FIG. 11, due to the fact that mattress cover 16 will forceweight distributing sheet 15 and the softer sheet 17 on top towards theflat edges of water mattress bag 1, and main mattress member 3,respectively. Since said components, and the main mattress member 3itself consist of resilient springy materials, said components will beslightly compressed by mattress cover 16 causing said increase of thehydrostatic pressure in water mattress bag 1. Said increase of thehydrostatic pressure will cause reduced sag in case of a load ascompared to what is shown in FIG. 10, because said increased hydoostaticpressure must be overcome by part of the load before sagging orcompression occurs. Simultaneously with said compression of watermattress bag 1 water will be displaced to the marginal areas of watermattress bag 1, where the swelling of the expansion portion 1c of saidbag is partly subdued because of the stretched mattress- cover 16. Also,the formed swelling will partly exert a downwards directed force ontothe flat edges 3b of main mattress member 3, which will yield in aspringy manner, as shown in FIG. 12, and will exert a counter pressureonto water mattress bag 1 contributing to said reduction of sagging.

A water mattress has thus been achieved which has a good bearingcapacity in relation to the used volume of water in water mattressbag 1. Said volume of water in a water mattress having a width of 75 cmmay be as small as approximately 50 liters. This means that the bearingcapacity of a water mattress acording to the invention as disclosedabove is sufficient even though the used volume of water in said watermattress bag is less than the volume of water which the user's weightwould displace.

When water is maintained under an increased hydrostatic pressure, asmentioned above, it has no space for developing wave crests, and therewill thus be no so called "wave ripples" when the mattress is used.

Weight distributing sheet 15 will settle as a further wave subduingmeans "on top of the water", i.e. on top of water mattress bag 1 andwill also subdue any ripples.

Thus, a water mattress is provided which has a particularly low rippleforming effect without having any special components introduced into theinterior of water mattress bag 1 to calm any possible movement of waterthat might create ripples.

The above disclosed water mattress is, in addition to the conventionalsafety sheet 18 against leakage of water which is always provided indirect contact with bottom 1a of water mattress bag 1, provided with afurther safety sheet 19 to enclose the sides and bottom of main mattressmember 3. In case the conventional safety sheet 18 should be puncturedleaking water will be absorbed by main mattress member 3 and held backby further safety sheet 19. In this manner a larger volume is, thus,encased and will be able to absorb the considerably smaller volume ofwater in water mattress bag 1 in case of a "total wreckage".

In order to secure that mattress cover 16 can adjust said increasedhydrostatic pressure it may, e.g. be provided with side walls 16a of amaterial of high tensile strength, and if desired, bottom 16b and/or top16c may be made from a material with tensile strength. Combinations of aresilient material and a material with tensile strength are possible aswell in order to ensure desired effects when water mattress 10 isloaded. If a material with tensile strength is, e.g. used in the sidewalls 16a of said mattress cover 16, they will highly contribute toreduce said sausage-like swelling 1c of the marginal areas of watermattress bag 1. If a material with tensile strength is used in theentire mattress cover, this will contribute to more compression of mainmattress member 3, and weight distributing sheet 15 as well as thesofter sheet 17 when loaded, because water in said water mattress bag 1will not be able to flow to expansion portion 1c to the same degree,thus, causing a more firm surface to lie on.

I claim:
 1. A water mattress bag of a resilient material and providedwith a valve (2) for filling/draining water, for use with a mainmattress member (3) of a resilient, flexible foamed plastic or someother similar material having a recess in its surface (4) extending overa substantial portion of the length (L) and width (B) of said surface,the bottom side (1a) of said water mattress bag (1) being shaped with abulge (1a') for cooperation with said recess (4), and the top side (1d)of said water mattress bag (1) having a length (L) and width (B),saidwater mattress bag (a) consisting of a blank (A) of a watertightresilient foil/sheet material with tensile strength with two pairs ofopposed lateral edges (1b,1b) and (1c,1c), one pair of lateral edges(1b,1b) having curved or tongue shaped projections (1b,'1b'), with saidlateral edges with curved projections joined together along alongitudinal seam (5) and forming a sleeve (A') the openings of whichare limited by a second pair of lateral edges (1c,1c) and closed bytransverse welded seams in superimposed sleeve walls flattened againstone another with said longitudinal seam (5) crossing the centralportions of said transverse welded seam (6,6), said transverse weldedseams (6) being provided at a distance from the ends of sleeve (A')having lateral edges (1c,1c).
 2. A water mattress bag as defined inclaim 1, characterized in that said transverse welded seams (6,6) areprovided between the ends of said projections (1b',1b') and the secondpair of lateral edges (1c,1c).
 3. A water mattress bag as defined inclaim 1, characterized in that said transverse welded seams (6) arecurved towards each other at their end portions to create a softtransition into the longitudinal sides of said water mattress bag.
 4. Awater mattress bag as defined in claim 1, characterized in that thedistance between said transverse welded seams (6,6) is approximatelyequal to the length (7) of main mattress member (3), for folding the endportions of said water mattress bag (1) with said transverse weldedseams (6,6) under said water mattress bag in a position for use on mainmattress member (3).
 5. A water mattress, comprising a flexible mainmattress member (3) with a recess (4) curved in the transverse andlongitudinal directions in its top face with flat edges (3b) about saidrecess,a water mattress bag (1) having a bottom (1a) with a shape thatconforms to said recess (4), and a flat top (1d), said water mattressbag resting on said main mattress member and partly within said recess,a flexible sheet (15) on top of said water mattress bag (1) and amattress cover (16), said water mattress bag (1) being partly filledwith water and extending to the lateral edges (3a) of main mattressmember (3) thus covering the flat edges (3b) of main mattress member (3)around recess (4), said sheet (15) being a weight distributing sheet fordistributing weight when loaded, and being shaped complementary with the(4), and having a thickness (a) casing the top (15a) of said weightdistributing sheet (15) to project at least up to the flat edges (3b) ofmain mattress member (3) in a non-loaded state, said mattress cover (16)being stretched around main mattress member (3) with said components (1,15) to keep them in place, and to cause a bias/increase of pressure inthe water inside water mattress bag (1) in a non-loaded state of saidwater mattress (10), said water mattress bag (1) consisting of a blank(A) of a watertight resilient foil/sheet material with tensile strengthwith two pairs of opposed lateral edges (1b,1b) and (1c,1c), one pair oflateral edges (1b,1b) having curved or tongue shaped projections(1b',1b'), with said lateral edges with curved projections joinedtogether along a longitudinal seam (5) and forming a sleeve (A') theopenings of which are limited by a second pair of lateral edges (1c,1c)and closed by transverse welded seams in superimposed sleeve wallsflattened against one another with said longitudinal seam (5) crossingthe central portions of said transverse welded seam (6,6), saidtransverse welded seams (6) being provided at a distance from the endsof sleeve (A') having lateral edges (1c,1c).
 6. A water mattress asdefined in claim 5, characterized in that a flexible sheet (17)extending outwards to the lateral edges (3a) of main mattress member (3)and being softer than said weight distributing sheet (15) is providedbetween said mattress cover (16) and said weight distributing sheet(15).
 7. A water mattress as defined in claim 5 characterized in thatthe side walls (16a) of said mattress cover (16) have tensile strength.8. A water mattress as defined in claim 7 characterized in that thebottom (16b) of said mattress cover (16) has also tensile strength.
 9. Awater mattress as defined in claim 7, characterized in that the top(16c) of said mattress cover (16) has tensile strength.
 10. A watermattress as defined in claim 6, characterized in that said weightdistributing sheet (15) and said softer, flexible sheet (17) are in onepiece.
 11. A water mattress bag of generally tubular shape (A') for usewith a main mattress member having a recess in its surface extendingover a substantial portion of the length and width thereof, the bottomside of said tubular water mattress bag being shaped with a bulgeconforming with the recess of the main mattress member, and the top sideof water mattress bag having a length and width approximately equal tothe length and width of the main mattress member,said water mattress bagbeing formed of a blank (A) of a water tight resilient foil-sheetmaterial having a pair of opposed transverse edges (1c) and a pair ofopposed longitudinally extending edges (1b), said longitudinallyextending edges being shaped with curved shaped projections (1b') joinedtogether along a longitudinal seam (5) to provide said bulge forcooperation with the recess of the main mattress member, the tubularwater mattress bag being flattened to define a top side (1d) and abulged bottom side (1a') with end portions in facing contact, and saidend portions being joined by lap-welded seams (6) at a distance spacedfrom the transverse edges (1c), said blank being formed into said watermattress bag by mutually displacing the top side thereof from the bottomside before said transverse lap welding seams are made in order to forma bottom that is longer than the top to achieve said longitudinal bulgeof the bottom.